This invention relates to hydronic heating systems for dwellings, offices, etc. and more particularly to such a system including a diverting valve in the system boiler water return line, adapted and operated to feed some of the system return water flow to the system boiler supply water flow so as to maintain the temperature of the supply water flow to the system heating loops within a predetermined range.
Hydronic heating systems for heating the rooms in a dwelling, office, etc, are used widely in Europe and to a lesser extent in the United States. Water heated in a boiler is distributed to heating loops of tubing in the dwelling that carry the heat by radiation, conduction and convection to the rooms in the dwelling. A common technique provides a boiler hot water supply feeding the supply header of the heating loops and the boiler water return to which the return header of the heating loops connects. The return water is heated in the boiler and sent out again as hot supply water, and so the water is cycled through the essentially closed system. One or more water pumps in this system keep the water flowing and valves control water flow rates through the loops depending on demand.
A heating loop may include several heating elements like wall mounted radiators and/or baseboard finned tubing that are the principal heat exchangers of the loop, or the tubing itself may be the principal heat exchanger of the loop. In the latter case the tubing is usually buried in the floor of a room and the tubing heats the floor. Often the tubing is buried in a special concrete and so heat exchange is principally by conduction and radiation to the concrete, which in turn heats the room by some conduction and convection, but principally by radiation. Hence, this type of heating is called Radiant Floor Heating (RFH). Similarly, the tubing is sometimes mounted in a wall against the material or panels that form the exposed surface of the wall and this type of heating is called Radiant Wall Heating (RWH).
In such RFH and RWA systems and other hydronic heating systems using wall radiators and/or baseboard finned tubing elements, the supply water temperature from the boiler must be controlled so that it does not exceed certain limits that are substantially lower than the usual boiler supply water temperature. There are several reasons for this: first, the temperature of radiator elements on the wall must not be so high that they are not safe to touch; second, for RFH and RWH the temperature of the floor or the wall must not be uncomfortable hot; and third, where the tubing is plastic, the water temperature for some plastic materials must not exceed about 140.degree. F. Good quality "cross-linked" polyethylene tubing, on the other hand, can carry water at temperature in excess of 140.degree. F. without any deterioration of the tubing or the tubing oxygen barrier.
The heating loop supply water temperature could be maintained low and so avoid these problems by simply operating the boiler at a lower water temperature. However, that can cause flue gas condensation on the boiler water heat exchanger. For example, the flue gas due point can be as high as 140.degree. F. and so to avoid flue gas condensation it is preferred that the boiler supply water temperature be not less than 140.degree. F.
In hydronic heating systems subject to such water temperature limitations, where the boiler is powered by burning fossil fuels, the boiler water supply temperature is usually well above 140.degree. F. and often at about 190.degree. F. to 200.degree. F., and so the boiler supply temperature must be stepped down before it is fed to the heating loops. In the past, a three-way, electrically controlled, motorized mixing valve has been used in the boiler supply line that feeds the supply header for the heating loops, between the boiler supply and the heating loops supply header. This mixing valve has two inputs and one output. One input is directly from the boiler hot water supply, the other input is from the return header of the heating loops and the output is directly to the supply header of the heating loops. The mixing valve motor is electrically energized by remote reset controls that sometimes respond to outside ambient temperature, inside room temperature, boiler water temperature, supply header water temperature, etc. In operation, the mixing valve mixes some return water with the hot supply water to reduce the temperature of the supply water that is fed to the supply header of the heating loops. Such prior systems perform quite satisfactorily, but they are relatively expensive, require remote transducers and electric power to the valve's motor and relatively greater skill to install and adjust for efficient operation.
In an effort to reduce expense, non-motorized mixing valves have been used in the boiler supply line. These have the disadvantage of providing less comfort and lower long term fuel economy. However, for the small installation (kitchen-bath additions, etc. to a dwelling), where it is difficult to justify the cost of a more sophisticated motorized valve and its controls, these systems are sometimes used. They usually have a remote electrically operated room thermostat controlling a circulator wired through a surface aquastat to prevent overheated water from entering the heating loops; and on the boiler supply line is a dial thermometer that indicates the supply water temperature into the loop supply header. However, manually setting the water temperature into the heating loops by adjusting the valve setting is not precise. Often within a few hours after start-up, when temperatures throughout the system have stabilized, fluctuations of the boiler supply water temperature, or varying load conditions at other parts of the system will cause excessive fluctuations of water temperature delivered by the valve to the heating loops supply header. These systems have no feedback control to the mixing valve that is derived from the heating loop supply header water temperature.
An improved system using a three-way diverting valve in the system return line is described in co-pending U.S. patent application Ser. No. 545,339, filed Jun. 28, 1990 by the inventor herein and entitled "Hydronic Heating Water Temperature Control System. In that patent application a three-way, modulated diverting or by-pass valve is provided in the return line to the boiler, between the heating loop return header and the boiler return. The diverting valve has one input and two outputs. The input is from the heating loops return header, the first output is to the boiler return line and the second output is to the boiler supply line. The diverting valve diverts some of the cooler return water to the hot supply water to reduce the temperature of the supply water feeding the heating loop supply header. Thus, the supply water is diluted with return water, lowering the temperature of the supply water directly from the boiler. The diverting valve is a modulated valve and the temperature of the supply water flowing to the supply header is detected and used as a feedback control signal to modulate the valve.
In that system, it is preferred that the water pump be in the return line between the return header and the diverting valve input, so that the diverting valve input is at the high pressure side of the pump. However, the water pump can, instead, be located between the by-pass tee connector in the supply line and the heating loop supply header.
This use of a diverting valve in the return line with the feedback control affords a technique of "Set Point Control". The three-way diverting valve in the return line with its control, including temperature feedback from the heating loops supply header, provides automatic water tempering, insuring constant supply water temperature to the heating loops. It may be relatively inexpensive and reliable and can be the primary entry-level controller in a hydronic heating system in a dwelling, office, etc. High quality three-way modulated diverting valves are available from a number of sources.
The feedback control (set point control) of the diverting valve can be provided by remote electric transducers and a motor driving (modulating) the diverting valve. Feedback control can also be provided by a non-electric thermostatic actuator head that engages the diverting valve stem and is controlled by a capillary temperature sensor. In both cases, the feedback control derived from the temperature of the diluted (tempered) supply water that is fed to the heating loops header can be the primary valve modulation control.
A typical three-way diverting valve such as shown in said co-pending U.S. patent application Ser. No. 545,339 is a conventional three-way diverting valve. It has one input and two outputs; the through output is to the return line to the boiler; the diverted output is to the boiler supply line; it has a spring loaded valve stem that carries two plugs, one to close the through output and the other to close the diverted output; and a spring that urges the stem to move in the direction that closes the diverted output and opens the through output. Thus, in view of the intrinsic operation of any control head, whether it is electric, thermostatic or otherwise, the control head piston engages the valve stem only to push the stem in and so increase diverted water flow when the thermostat control head setting calls for less heat or a lower temperature of the heating loop header water temperature. If the head is set too high, or does not work, or is removed from the valve, the diverted output closes and the supply water to the heating loops is not diluted and may be too hot for comfort of cause other undesirable effects mentioned herein.
As mentioned above, any type of electric or thermostatic control head attached to the diverting valve as described herein, is such that the control head piston engages the valve stem only to push the stem in and so increase diverted water flow when the control head setting (set electrically or mechanically) calls for less heat or a lower temperature of supply water to the supply header than the actual temperature of supply water to the supply header (actual heat). The actual temperature of supply water to the supply header is referred to herein as the control water temperature and if that temperature is below what the head setting calls for, the head piston does not engage the valve stem at all and so the valve spring moves the stem to close the diverted output and open the through output.
Thus, this valve control actuation produces: maximum flow of diverted return water to the supply line when the control water temperature is substantially greater than the control head setting; a variable modulated flow of diverted return water to the supply line when the control water temperature and the control head setting are about the same (equivalent); and zero diverted water flow when the control water temperature is substantially less than the control head setting. Here, even when the head is functioning properly, a situation can arise when there is no diverted water flow, and so the header for the heating loops is fed undiluted boiler supply water, which is often too hot for efficient delivery of heat and too hot for comfort. As mentioned above, the same problem arises if the control head is removed from the valve, in which case the valve spring also completely closes the diverted water output and the header for the heating loops is fed undiluted boiler supply water making: radiator elements on the wall too hot to touch; RFH floor and RWH wall temperatures uncomfortable hot; and in heating loops using plastic tubing, the water temperature would be in excess of the recommended temperature and could cause deterioration of the tubing or the tubing oxygen barrier.